The present invention relates to electroluminescent devices. Examples of electroluminescent devices include small molecule organic light emitting devices (SMOLED), polymer light emitting devices (PLED), and inorganic electroluminescent devices. The term “organic light emitting devices (OLED) refers to both small molecule organic light emitting devices and polymer light emitting devices.
A typical prior art electroluminescent device comprises a transparent substrate (which is 1 to 4 orders of magnitude thicker than the remaining layers), a transparent first electrode layer, a light-emitting element including at least one light-emitting layer, and a reflective second electrode layer. Light is generated in the electroluminescent device when electrons and holes that are injected from the two electrodes flow through the light-emitting element and generate light by either recombination or impact ionization. The light-emitting element can include several layers of materials including at least a light-emitting layer where the emitted light is generated. In the case of an OLED device, for example, the light-emitting element can include an electron injection layer, an electron transport layer, one or more light-emitting layers, a hole transport layer, and a hole injection layer. One or several of these layers can be combined and additional layers such as electron or hole blocking layers can be added. Most frequently, the first electrode layer is the anode and the second electrode layer is the cathode.
Furthermore, OLED structures called stacked OLED (or tandem OLED or cascaded OLED), are formed by stacking several individual OLEDs vertically. Forrest et al. in U.S. Pat. No. 5,703,436 and Burrows et al. in U.S. Pat. No. 6,274,980 disclosed their stacked OLEDs. In their disclosures, the stacked OLEDs are fabricated by vertically stacking several OLEDs, each independently emitting light of a different color or of the same color. However, each OLED unit in their devices needed a separate power source. In an alternative design, a stacked OLED structure, which is fabricated by stacking several individual OLEDs vertically and driven by only a single power source, was disclosed (see U.S. Pat. Nos. 6,337,492, 6,107,734, 6,717,358, U.S. Patent Application Publication Nos. 2003/0170491 A1, 2003/0189401 A1, and JP Patent Publication No. 2003045676A). In a stacked OLED having a number of N (N>1) EL units, the luminous efficiency can be N times as high as that of a conventional OLED containing only one EL unit (of course, the drive voltage can also be N times as high as that of the conventional OLED). Therefore, in one aspect to achieve long lifetime, the tandem OLED needs only about 1/N of the current density used in the conventional OLED to obtain the same luminance although the lifetime of the tandem OLED will be about N times that of the conventional OLED. In the other aspect to achieve high luminance, the tandem OLED needs only the same current density used in the conventional OLED to obtain a luminance N times as high as that of the conventional OLED while maintaining about the same lifetime. Each organic EL unit in a tandem OLED is capable of supporting hole and electron-transport, and electron-hole recombination to produce light. Each organic EL unit can comprise a plurality of layers including HTL (hole transport layer), ETL (electron transport layer), LEL (light emitting layer), HIL (hole injection layer), and EIL (electron injection layer). A light-emitting layer (LEL) can comprise one or more sub-layers each emitting a different color.
It is also common to employ one or more techniques for extracting light which is trapped in the high index OLED and substrate materials to the air. Various techniques have been suggested to increase the efficiency of the thin-film electroluminescent devices by reducing the light trapping effect and permit the substrate-mode and organic-mode of light to emit from the device. These techniques are described in the following references: U.S. Pat. Nos. 5,955,837, 5,834,893; 6,091,195; 6,787,796, 6,777,871; U.S. Patent Application Publication Nos. 2004/0217702 A1, 2005/0018431 A1, 2001/0026124 A1; WO 02/37580 A1, and WO02/37568 A1.
It is also known to connect a plurality of individual OLED devices in series and in parallel in order to form a 1 or 2 dimensional array of adjacent emitting devices. In U.S. Pat. No. 6,693,296, Tyan describes a structure in which adjacent OLED segments are connected in series on a single substrate. In U.S. Pat. No. 6,515,417, Duggal describes a method of mounting a plurality of individual OLED devices on a common substrate in order to form a larger area panel. The pairing of two OLED strips oriented in opposite directions, with the terminal anode of one strip connected to the terminal cathode of the second strip, and vice versa so that when driven by an AC signal, they will light alternately is disclosed by Cok in U.S. Pat. No. 7,034,470 and by Duggal in U.S. Pat. No. 6,800,999. In both of these references, the individual segments or devices are wired anode to cathode on each end, forming a rectifier such that the two strings will light alternately during the ac cycle.
A problem with all of the devices described thus far is that they provide uniform output only if the lateral extent of the individual devices is small. This is not significant in the case of small displays where the individual pixels are typically much less than 1 mm in extent. The problem becomes more noticeable for large displays, and for non-pixelated devices such as fixtures for general lighting and backlights for liquid crystal displays. If an electroluminescent segment is large, the current flowing through the transparent electrode will cause a significant voltage drop across that electrode which will, in turn cause a variation in the current density flowing through the device, resulting in a variation of the device brightness.
This problem is illustrated in FIG. 1, which depicts a prior art electroluminescent segment, referred to here as an EL-segment 100 configured as a bottom emitter. The device includes a transparent substrate 110, a transparent anode 120 such as ITO, a light emitting-element, 130, and a reflective cathode 140, such as Al or Ag arranged as shown in FIG. 1. When the left edge of the anode 120 is biased positive relative to the right edge of the cathode 140, an anode current 150 flows from left to right within the anode 120, and a cathode current 160 flows from left to right within the cathode 140. At each point along the device, a through device current 170 flows from the anode 120, through the electroluminescent layer, to the cathode 140, causing the generation of light. The local through device current density flowing through the electroluminescent layer depends on the voltage difference between the anode 120 and the cathode 140 at that point. However, the anode current 150 causes a voltage drop along the anode 120, which is much larger than the voltage drop along the more conductive cathode 140. As a result voltage across the electroluminescent layer is larger on the left side of the device shown in FIG. 1, than it is on the right side of the device. This results in the left side of the device being brighter than the right side of the device.
This non-uniformity can detract from the appearance of a light panel for use in general lighting or as a backlight for LCD. In direct-lit LCD backlights (as opposed to edge-lit LCD backlights prevalent in small laptop displays), it is common to utilize diffusers spaced at some distance from the luminescent features in order to improve the uniformity of the backlight. For instructive purposes, this is illustrated in FIG. 2, which illustrates two different light panels 200. The light panel 200 on the left has large discrete lamps 210 which are non-uniform in brightness as indicated by the shading. The light panel 200 on the right has smaller discrete lamps 210, which are also non-uniform in brightness. Each lamp array can be characterized by a parameter, which will be referred to as the “brightness non-uniformity extent” (BNUE) 220. The BNUE 220 is the distance over which the non-uniformity in brightness extends, and is smaller for the light panel 200 on the right.
Transmissive diffuser 230 is positioned in front of the discrete lamps 210, separated from the plane of the discrete lamps 210 by a diffuser gap 240. Plots 250, 260 and 270 show the brightness at the diffuser as a function of position when the diffuser gap is close to zero, small, and larger respectively. As the diffuser gap 240 is increased, the light panel brightness becomes more uniform. More importantly, the brightness uniformity of the light panel 200 with the smaller BNUE 220 is better at any non-zero size of the diffuser gap 240. Two properties, which are of value in an LCD backlight as well as in a general lighting panel, are uniformity and thinness. These are both improved when the BNUE 220 of the discrete lamps 210 in a light panel 200 is smaller.
In a two dimensional panel, the BNUE 220 will likely be different in the two orthogonal directions in the plane. The uniformity of a light panel 200 with a spaced diffuser will track most closely with the smaller BNUE 220. Therefore, the BNUE 220 of a two dimensional light panel would be the smallest associated with any in-plane direction.